Induction with high defibrillation threshold

Tracing

1. 8 stimuli at a rate of 150 bpm;
2. low-amplitude (1.1 Shk) shock delivered on the T wave;
3. after the refractory period, the first ventricular event (--) was not counted;
4. a polymorphic ventricular tachyarrhythmia alternating initially between sinus (VS) and VF (VF) zones was triggered;
5. the initial rolling detection window began only after the detection of 3 short consecutive cycles. In this case, the first 2 short cycles (VF) were followed by 1 VS. Consequently, the 3 short consecutive cycles were missing and the initial detection window did not begin;
6. third consecutive cycle in the VF zone; 
7. after 8 cycles in the VF zone (including the first 3 consecutive cycles) with 2 interposed VS cycles, (fulfilling the of 8 out of 10 criterion), an episode of VF  (V-Epsd) was detected;
8. the first cycle following the end of the Duration (1 sec) is a VS, explaining the presence of the “V-Dur” marker (while the 6 out of 10 criterion remains fulfilled, the V-detect marker can only appear after a short cycle;
9. on the first VF cycle following the end of the Duration (with the 6 out of 10 short cycles criterion remaining fulfilled), a sustained episode was detected, explaining the “V-detect” marker; onset of the capacitors charge;
10. end of charge;
11. re-confirmation at the end of charge: the next first cycle following the charge was not counted (--), the next 2 were in the VF zone, fulfilling the 2 out of 3 short cycles criterion;
12. delivery of a 17 J electrical shock;
13. refractory period following the shock; the first ventricular cycle that followed was not counted;
14. the electrical shock was unsuccessful and the arrhythmia continued. After an electrical shock, a re-detection phase begins, applying the same criteria for ventricular detection from earlier;
15. from this cycle onward, the 8 out of 10 short cycles criterion was fulfilled (no visible marker on the tracing); onset of the post-shock Duration (1 sec);
16. re-detection of VF (8 out of 10 cycles in the VF zone + 1 sec post-shock Duration) and repeat charge of the capacitors;
17. end of charge;
18. re-confirmation: the first cycle after the charge was not counted (--); the next 2 cycles were in the VF zone; the of 2 out of 3 short cycles criterion was fulfilled; a 26-J electrical shock was delivered;
19. a 500-ms refractory period followed the shock; the next ventricular cycle was not counted;
20. the electrical shock was unsuccessful and the arrhythmia continued;
21. artifact caused by the shock delivered by the external defibrillator (ordered by the operator);
22. termination of the arrhythmia and several ventricular paced events.

Graph

The graph illustrates a few undersensed R waves (23) and shows the delivery of 2 unsuccessful electrical shocks (24 and 25).